PROJECT SUMMARY/ABSTRACT: Many deleterious human mutations cause genetic diseases with incomplete penetrance: only some mutant individuals manifest a disease phenotype. The long-term goal of this proposal is to understand how a deleterious mutation has devastating disease consequences in some individuals, but others are able to overcome a mutation and live normal healthy lives. We manipulated mutant penetrance upward and downward in a zebrafish craniofacial disease model by selective breeding. In our high- penetrance line, a mutation in the transcription factor encoding gene mef2ca causes severe, lethal craniofacial phenotypes, while in the low-penetrance background animals overcome the same deleterious mutation and grow to be viable, fertile adults. By comparing high- and low-penetrance strains we will discover the mechanisms of genetic resilience, defined here as the as the ability of a mutant organism to overcome genetic perturbation and achieve a wild-type phenotype. Three specific aims test the overarching hypothesis that cell populations use multiple penetrance modifier mechanisms to different extents to overcome a deleterious mutation: (Aim 1) compensatory signaling pathways hypothesis. We found the BMP and Notch signal transduction pathways are differentially active between the low and high-penetrance strains. Genetic and gain- and loss-of-function tools and live imaging will test if these pathways functionally modify penetrance by activating mef2ca target genes in the absence of mef2ca function. (Aim 2) paralogous compensation hypothesis. We observe that some members of the mef2 family of transcription factors (paralogs) are modestly upregulated in the low penetrance strain. Genetic misexpression will test if paralogs dose-dependently contribute to penetrance and the degree to which mef2 paralogs are interchangeable during craniofacial development. (Aim 3) modifier specificity hypothesis. The mef2ca mutation is associated with phenotypes in several craniofacial skeletal elements, and we observe that the penetrances of the different craniofacial phenotypes change to different extents in our selectively bred backgrounds. We use single-cell transcriptomics and genome editing to understand how specific craniofacial progenitor cell populations overcome the mef2ca mutation to greater or lesser extents. A better understanding of how animals overcome deleterious mutations could lead to novel therapeutic approaches to managing disease phenotypes.